It is known that, among the weft feeders for looms, there are some--particularly used for jet looms--which measure predetermined weft yarn lengths by means of optical units. Usually, said devices (called "measuring weft feeders") also provide to stop the measured weft yarn lengths by means of magnets, whose cores block the weft yarn in correspondence of the conical end of the drum around which the yarn reserve is wound. A particularly efficient device of this type is that described in the Italian Patent Application No. 20307 A/83 of the same Applicant, to which reference is made for fuller details.
In devices of the type mentioned heretofore--and particularly in that of the cited Italian Application--the magnets positively guarantee the stopping of the measured yarn when said yarn is under tension (which is of course a very slight tension). However, according to the Applicant's most recent experiments, this condition is not always fulfilled; in fact, during operation of the loom and of the weft feeder applied thereon, there are times and circumstances in which even the slightest weft yarn tension, required for the proper working of the device, is missing.
It has actually been established that the condition of slightly tensioned weft yarn is usually fulfilled while the weft is being inserted in the loom, in that the air jets from the main and secondary nozzles are apt to supply said tension. Whereas, a very delicate step of the weft insertion in a jet loom, is when the yarn is being cut by the cutting devices positioned at the outlet of the main nozzle.
Referring to FIG. 1 of the accompanying drawings--which schematically shows the weft insertion steps in a jet loom--it can be seen that the weft is drawn by the measuring weft feeder 1 and is inserted into the shed thanks to the main nozzle 2, which is positioned on the loom sley, aligned with the reed and movable therewith.
Weft insertion practically starts in the position A, in which the reed is fully open, and it ends about half way between said position and the position B, in which the reed is fully closed (reed beat-up position). As known, the end of the insertion corresponds to the measuring of the unwound weft yarn, that is, to the blocking of the yarn by the movable body of the energized magnet. Subsequently, close to the reed beat-up position, the weft yarn is cut by the cutter F positioned between the main nozzle 2 and the reed P and movable therewith.
As can be seen from FIG. 1, in order to obtain short insertion times, the yarn guide at the outlet of the measuring weft feeder should be more or less aligned with the reed in position A (fully open reed), in order to prevent tensions on the yarn deriving from friction on the yarn guides. When cutting takes place, with the reed in position B (fully closed reed), the weft yarn is under tension along the stretch between the outlet of the measuring weft feeder and the fabric being woven, in that it is blocked at these two ends slightly before the reed reaches the beat-up position B.
The weft is in fact blocked in the weft feeder thanks to the magnet, and in the loom thanks to the closing of the shed.
The excess of tension on the weft is thus determined by the longer path it has to follow from the moment in which it gets blocked at the ends to the moment in which it gets cut. When cutting takes place, there is a sudden fall in the tension which causes oscillations along the stretched weft yarn, said oscillations travelling from the cut end of the yarn up to the yarn guide at the outlet of the weft feeder, and into said feeder, up to involving the yarn turns wound on the drum.
These oscillations--which turn into strong transversed and longitudinal waves--prevent the weft yarn from being slightly tensioned during the aforementioned working step, in which the yarn is instead subjected to strong oscillations; this could easily lead the yarn to be wedged under the magnet pin even after having stopped against said pin.
This behavior determines a wrong measuring of the inserted weft yarn, with errors which can take up different aspects, but all of which are always dangerous.
Usually, a yarn turn passes under the magnet pin (in special cases, it can be more turns) after the weft has been inserted, with the result that--in the case of working with a single-colored loom--one finds oneself in the presence of a yarn length longer by a turn in the next insertion, and--in the case of working with a multi-colored loom--one may be faced with the simultaneous insertion of two turns of different-colored yarn.
This phenomenon--which takes place especially in the case of scarcely elastic yarns, wherein the effect produced by the sudden fall of tension cannot be absorbed by the elasticity of the yarn--is particularly harmful in the case of working with a multi-colored loom, in that it can occur on changing of the color, and it will appear as an insertion corresponding to one or more turns, simultaneously with the next color and in the same shed, which cannot be detected by the weft control systems used at present on jet looms.
It has already been observed that these drawbacks are reduced, up to almost disappearing, if the measuring weft feeder is positioned in respect of the loom so as to always produce a friction on the outlet yarn guide, or else if a similar friction is produced on a guide or bar, apt to deviate the yarn, being positioned between the outlet of the weft feeder and the inlet of the main nozzle. This arrangement would however involve such long insertion times as to be unacceptable, whereby, up to the present, no solution has yet been found to the problem of preventing, during the cutting step, weft yarn oscillations from travelling up beyond the outlet yarn guide, into the weft feeder, without varying the loom insertion times.